The present invention relates to calibration of a radiometer with reference temperatures from a noise source and, more particularly, to calibration of a radiometer with reference temperatures from an electronically adjustable noise source providing hot thermal radiation temperature from an output port and cold thermal radiation temperature from an input port.
Radiometers are used to measure thermal radiation or brightness temperatures emitted from a segment of a remote object. The segment is commonly referred to as a scene and may be a portion of the earth""s surface. Like most sophisticated instrumentation, radiometers require periodic calibration to insure accurate measurements. In practice, at least two known calibration temperatures that abound the brightness temperatures of the scene are used to calibrate a radiometer receiver. The lowest and highest calibration temperatures are referred to as cold and hot thermal radiation temperatures, respectively.
Radiometers are generally ground-based, airborne or satellite-based systems that measure brightness temperatures in the mostly cold range of 10xc2x0 K-300xc2x0 K. There are also specialized radiometer applications where an instrument is needed to measure hot brightness temperatures from forest fires and burning dumps. For these applications the radiometer must measure brightness temperatures in the range of 300xc2x0 K to greater than 1000xc2x0 K. The ground-based systems may utilize closed cycle refrigeration such as a sterling cycle cooler with liquid nitrogen or is liquid helium to generate cold thermal radiation temperatures xe2x80x9cTcxe2x80x9d. The closed cycle refrigeration systems are not considered practical for the satellite-based systems.
Referring to FIGS. 1-3, there are illustrated three traditional satellite-based systems for measuring the brightness temperature xe2x80x9cTaxe2x80x9d emitted from a portion of the earth""s surface and received by an antenna 36. The brightness temperature xe2x80x9cTaxe2x80x9d is then transmitted through an antenna feed 32 on an antenna-earth scene line 12 to a radiometer receiver 16 of the radiometer 150. Currently, satellite-based systems use calibration techniques that are either externally-based (FIGS. 1 and 2) or internally-based (FIG. 3).
Referring to FIG. 1, there is illustrated an externally-based calibration technique known as the sky horn approach. The sky horn approach utilizes a radiometer 150 which includes a first RF switch 10 connected to either the antenna-earth scene line 12 or a calibration line 14 to the radiometer receiver 16. In the calibration line 14 a second RF switch 18 alternately switches between a sky horn 20 and in internal warm load 22. The sky horn 20 outputs the cold space thermal radiation temperature xe2x80x9cTc,xe2x80x9d approximately 2.7xc2x0 K, and the internal warm load xe2x80x9cTw,xe2x80x9d approximately 300xc2x0 K. A precision thermistor 24 in thermal contact with the warm load 22 outputs an electrical hot thermal radiation temperature xe2x80x9cTdxe2x80x9d that is equivalent to the hot thermal radiation temperature xe2x80x9cTw.xe2x80x9d The electrical hot thermal radiation temperature xe2x80x9cTdxe2x80x9d is utilized in the calibration of the radiometer receiver 16.
The sky horn approach is a complex and expensive way to calibrate the radiometer receiver 16. The main problem is that the antenna-earth scene line 12 and calibration line 14 are separate lines, thereby requiring precise knowledge of the RF losses, mismatch losses and physical temperatures of each line to accurately calibrate the radiometer receiver 16. Also, the use of the sky horn 20 adds to the complexity of the calibration, because of possible interference of the sky horn pattern by a spacecraft or contamination caused by the earth or sun.
Referring to FIG. 2, there is illustrated another externally-based calibration technique for satellite-based systems using an antenna scanner 26. The antenna scanner 26 is a mechanical mechanism employed during a calibration mode to alternately couple a reflector plate 28 or an absorption target 30 to respectively feed a cold thermal radiation temperature xe2x80x9cTcxe2x80x9d or a warm thermal radiation temperature xe2x80x9cTwxe2x80x9d to the antenna feed 32. The antenna feed 32 is connected to the radiometer receiver 16. During an antenna mode when the brightness temperature xe2x80x9cTaxe2x80x9d is measured the antenna scanner 26 connects the antenna-earth scene line 12 to the radiometer receiver 16. The antenna scanner 26 does have an advantage over the sky horn approach in that only one RF path is utilized. However, the antenna scanner 26 is complex, bulky and adds significant size and weight to the radiometer 150.
Referring to FIG. 3, there is illustrated an internally-based calibration technique that may be used in a satellite-based system. The internal approach is very similar to the sky horn approach discussed previously and illustrated in FIG. 1. However, the internal technique may utilize a thermoelectric cooler 34 to generate a cold thermal radiation temperature xe2x80x9cTcxe2x80x9d of approximately 270xc2x0 K, instead of the sky horn 20 used in the sky horn approach. However, the warm and cold thermal radiation temperatures xe2x80x9cTcxe2x80x9d and xe2x80x9cTwxe2x80x9d used in the internal is approach may only be 30xc2x0 K apart. The 30xc2x0 K difference between the cold and warm thermal radiation temperatures xe2x80x9cTcxe2x80x9d and xe2x80x9cTwxe2x80x9d does not cover the full range of each brightness temperatures which are approximately 100xc2x0 K to 300xc2x0 K, (exclusive of burning materials) therefore, measurement accuracy of the radiometer receiver 16 will likely degrade below the cold thermal radiation temperature xe2x80x9cTc.xe2x80x9d
Accordingly, there is a need for an adjustable calibration noise source to provide cold to hot thermal radiation temperatures from a waveguide or coaxial port. There is also a need to provide a noise source manufactured using microwave integrated circuit (MIC) and/or monolithic microwave integrated circuit (MMIC) technologies. These and other needs are satisfied by the adjustable calibration noise source of the present invention.
The present invention is a radiometer calibration system utilizing an electronically adjustable noise source and a method for calibrating a radiometer. The noise source includes a transistor configured as a noise equivalent circuit having a gate port, drain port and source port. A source inductance providing series feedback for the noise source has one end coupled to the source port of the noise equivalent circuit and another end connected to ground. A bias circuit controls the amount of DC bias applied to the noise equivalent model. In order to match the impedances in the noise source, an output impedance matching network is connected to the drain port and an input impedance matching network is connected to the gate port of the noise equivalent model. The output and input impedance networks have an output port and input port, respectively. The noise source terminates a matched load to the output port while an adjustable cold thermal radiation temperature is generated at the input port. Alteratively, a port switch may be used to terminate a matched load to the input port while an adjustable hot thermal radiation temperature is generated at the output port.
According to the present invention there is provided an adjustable noise source for calibrating ground-based, airborne, or satellite-based radiometers.
Also in accordance with the present invention there is provided a noise source that functions in the millimeter and microwave spectrum.
Further in accordance with the present invention there is provided a noise source implemented as an integrated circuit.
Further in accordance with the present invention there is provided a calibration system having a noise source for measuring the radiometer receiver transfer function or receiver linearity.
Further in accordance with the present invention there is provided a calibration system having a noise source with a built-in-test capability providing noise figure measurements.
In accordance with the present invention there is also provided a radiometer having adjustable calibration time intervals to maximize the measurement of earth scenes.